Winchester disk drive units having one or more rotatable memory storage disks mounted within a sealed disk drive housing along with associated electromagnetic heads for reading and writing data on appropriately prepared disk surfaces are the most common and popular form of data storage for personal computers. Within the sealed disk drive, rotating constant speed spindle motor is provided for mounting and rotating the disks. Because of the extremely fine tolerances in the internal components to the disk drive, especially the head and disk surface, the head disk assembly is sealed against outside air to prevent entry of contaminants. In order to minimize the size of the disk drive, modern spindle motors are designed with many components inside the hub which supports the disks. Therefore, obviously, it is also critical to provide a seal between the spindle motor which mounts the disks for rotation and the interior of the head disk assembly. Magnetic fluid seals have been commonly adopted in spindle motors for disk drives as a means to seal either the external components from the inside of the drive, thus preventing external contaminants such as dust particles from entering the head disk assembly; or are used to seal portions of the regions within the head disk assembly to prevent the flow of air through the motor bearings, thus preventing particles such as grease containing aerosols from circulating from the motor into the interior of the head disk assembly. Some motors have single seal and labyrinth at opposite ends of the space to be enclosed; other motors have two magnetic seals.
A typical magnetic seal is shown in a simplified form in FIG. 1, with the seal 5 being mounted between the hub or housing 1 and the shaft 3. The hub and shaft are rotating relative to one another. A pair of annular pole pieces 13 and 15 are provided, sandwiching a permanent magnet 11 which is integrally inserted between the pole pieces to form the seal of the device. A ferrofluid 9, which is electrically and magnetically conductive, e.g., CFF200A from Ferrotech, is disposed between the pole pieces 13 and 15 and the shaft 3. The magnet 11 causes the ferrofluid to be retained in place between the pole pieces 13 and 15 and the shaft 3 so that a seal is formed through which contaminants cannot pass. Now, with the adoption of magnetoresistive "MR" heads in disk drives, a related problem has been identified which requires modification of accepted magnetic seal designs. Specifically, such magnetoresistive heads are extremely sensitive to the buildup of any static electrical charge on the rotating disks, as well as to contamination. Therefore, it is extremely important that in MR head drives, the pole pieces 13, 15 need to be electrically connected to each other, or both need to be in electrical contact with the hub/bearing holder assembly so that any electrical charge buildup on the disks can be safely conducted through the pole pieces from the hub to the shaft, and then to ground while maintaining the integrity of the magnetic seal. In magnetic seal design as known in the prior art, such electrical conductivity typically is not established to the extent desired. Typically, only one of the two pole pieces is electrically in contact with the hub or bearing assembly 1. Further, the magnet 11 is typically formed of a nonelectrically conductive material, so that the electrically conducting pole pieces 13, 15 are isolated one from another by the magnet 11. Moreover, although in the diagrammatic FIG. 1, the ferrofluid seal is shown in equal amounts 9 between the pole pieces and the shaft, in fact a majority of the fluid is collected between one pole piece and the shaft, primarily as a function of the direction from which the seal was filled with the fluid. As a result, the pole piece which has the lesser amount of fluid 9 between pole piece and shaft is in very poor electrical contact with the rotating shaft. Thus, magnetic seals of present designs provide inadequate contact between the outer hub and the shaft to successfully ground out any stray electrical charge.